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We improve thermal tolerance of a Co/Pt based synthetic ferrimagnetic (SyF) reference layer in a CoFeB/MgO-based perpendicular anisotropy magnetic tunnel junction by employing a surface modification treatment (SMT) using Ar plasma on a Pt buffer layer underneath the SyF layer. A higher perpendicular anisotropy and more robustness against annealing for the SyF reference layer is observed by the employment of SMT. From the cross-sectional scanning transmission electron microscope of the magnetic tunnel junction and atomic force microscopy on the Pt buffer layer, SMT has no significant effect on the surface roughness of the Pt buffer layer. On the other hand, X-ray diffraction analysis reveals that SMT increases the lattice spacing of the Pt buffer layer and shrinks that of the Co/Pt multilayer, which are maintained after annealing at 400 °C. Those structural changes could lead to the improvement of perpendicular anisotropy and thermal tolerance against annealing. A high-resolution Rutherford backscattering analysis reveals that Ar ions used for SMT penetrate to the Pt buffer layer, resulting in variation of the lattice spacing for the Pt buffer layer and Co/Pt multilayer. In addition, suppression of Fe diffusion from the CoFeB reference layer to the bottom Co/Pt multilayer is observed by using energy dispersive X-ray spectrometry, which also contributes to the higher thermal tolerance against annealing. The present observation reveals that the employment of SMT enables one to obtain both high perpendicular magnetic anisotropy and high robustness against annealing through crystallographic change of the Co/Pt multilayer and suppression of Fe diffusion from CoFeB layer into the Co/Pt multilayer. [ABSTRACT FROM AUTHOR]

In order to recover the patterning damage and improve the electric and magnetic properties of the patterned magnetic tunnel junctions (MTJs), the novel post-treatment process using oxygen ions has been studied. Generally, the oxygen is known as an unsuitable gas for the MTJs patterning because it causes degradation of the patterned MTJs by over-oxidation of MgO. By the way, if the damaged layer could be oxidized selectively without over-oxidation of the damage-less area, oxygen can be the most effective gas to recover the patterning damage of the MTJs. In this study, for the selective oxidation, we proposed the non-reactive oxygen treatment scheme called the oxygen showering post-treatment process (OSP) using an ozone diffusion chamber. By the specific OSP conditions, 8 l/min of the flow rate, 250 °C of the temperature, and 30 s of the time, the magneto-resistance (MR) was increased from 103% to 110%, and the switching current was decreased from 41.1 μA to 31.6 μA when compared with reference data at the same resistance level. These results show that the electric and magnetic properties of the patterned MTJs by the OSP treatment have been improved compared to the reference sample. The improvement in electric and magnetic properties by the OSP treatment is assumed because the reference sample already contains slight patterning damages at the edge of the MTJs despite the optimized patterning process, and these damages have been oxidized by the OSP treatment. Moreover, by the OSP treatment, the proportion of the electric short fail was dramatically decreased from 1.51% to 0%, which is a remarkable improvement in terms of a successful commercialization of spintronic devices. [ABSTRACT FROM AUTHOR]

To identify the degradation mechanism in magnetic tunnel junctions (MTJs) using hydrogen, the properties of the MTJs were measured by applying an additional hydrogen etch process and a hydrogen plasma process to the patterned MTJs. In these studies, an additional 50 s hydrogen etch process caused the magnetoresistance (MR) to decrease from 103% to 14.7% and the resistance (R) to increase from 6.5 kX to 39 kX. Moreover, an additional 500 s hydrogen plasma process decreased the MR from 103% to 74% and increased R from 6.5 kX to 13.9 kX. These results show that MTJs can be damaged by the hydrogen plasma process as well as by the hydrogen etch process, as the atomic bonds in MgO may break and react with the exposed hydrogen gas. Compounds such as MgO hydrate very easily. We also calculated the damaged layer width (DLW) of the patterned MTJs after the hydrogen etching and plasma processes, to evaluate the downscaling limitations of spin-transfer-torque magnetic randomaccess memory (STT-MRAM) devices. With these calculations, the maximum DLWs at each side of the MTJ, generated by the etching and plasma processes, were 23.8 nm and 12.8 nm, respectively. This result validates that the hydrogen-based MTJ patterning processes cannot be used exclusively in STTMRAMs beyond 20 nm. [ABSTRACT FROM AUTHOR]

The robustness of data load of metal-oxide-semiconductor/magnetic tunnel junction (MOS/MTJ) hybrid latches at power-on is examined by using Monte Carlo simulation with the variations in magnetoresistances for MTJs and in threshold voltages for MOSFETs involved in 90 nm technology node. Three differential pair type spin-transfer-torque-magnetic random access memory cells (4T2MTJ, 6T2MTJ, and 8T2MTJ) are compared for their successful data load at power-on. It is found that the 4T2MTJ cell has the largest pass area in the shmoo plot in TMR ratio (tunnel magnetoresistance ratio) and Vdd in which a whole 256 kb cell array can be powered-on successfully. The minimum TMR ratio for the 4T2MTJ in 0.9Vdd<1.9V is 140%, while the 6T2MTJ and the 8T2MTJ cells require TMR ratio larger than 170%. [ABSTRACT FROM AUTHOR]

A compact nonvolatile programmable switch (NVPS) using 90 nm CMOS technology together with perpendicular magnetic tunnel junction (p-MTJ) devices is fabricated for zero-standby-power field-programmable gate array. Because routing information does not change once it is programmed into an NVPS, high-speed read and write accesses are not required and a write-control transistor can be shared among all the NVPSs, which greatly simplifies structure of the NVPS. In fact, the effective area of the proposed NVPS is reduced by 40% compared to that of a conventional MTJ-based NVPS. The instant on/off behavior without external nonvolatile memory access is also demonstrated using the fabricated test chip. [ABSTRACT FROM AUTHOR]